To meet the demand for wireless data traffic having increased since deployment of 4th generation (4G) communication systems, efforts have been made to develop an improved 5th generation (5G) or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a ‘Beyond 4G Network’ or a ‘Post Long Term Evolution (LTE) System’.
The 5G communication system is considered to be implemented in higher frequency (mmWave) bands, e.g., 60 GHz bands, so as to accomplish higher data rates. To decrease propagation loss of the radio waves and increase the transmission distance, the beamforming, massive multiple-input multiple-output (MIMO), Full Dimensional MIMO (FD-MIMO), array antenna, an analog beam forming, large scale antenna techniques are discussed in 5G communication systems.
In addition, in 5G communication systems, development for system network improvement is under way based on advanced small cells, cloud Radio Access Networks (RANs), ultra-dense networks, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, Coordinated Multi-Points (CoMP), reception-end interference cancellation and the like.
In the 5G system, Hybrid frequency shift keying (FSK) and quadrature amplitude modulation (FQAM) and sliding window superposition coding (SWSC) as an advanced coding modulation (ACM), and filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) as an advanced access technology have been developed.
The rapid development of information industry, particularly the increasing demand from the mobile Internet and the Internet of Things (IoT), brings about unprecedented challenges in the future mobile communications technology. According to the ITU-R M. [IMT.BEYOND 2020.TRAFFIC] issued by the International Telecommunication Union (ITU), it can be expected that, by 2020, mobile services traffic may grow nearly 1,000 times as compared with that in 2010 (4G era), and the number of user device connections may also be over 17 billion, while a vast number of IoT devices gradually expand into the mobile communication network, and the number of connected devices may be even more amazing. In response to this unprecedented challenge, the communications industry and academia have prepared for 2020s by launching an extensive study of the fifth generation of mobile communications technology (5G). Currently, in ITU-R M. [IMT.VISION] from ITU, the framework and overall objectives of the future 5G have been discussed, where the demands outlook, application scenarios and various important performance indexes of 5G have been described in detail. In terms of new demands in 5G, the ITU-R M. [IMT.FUTURE TECHNOLOGY TRENDS] from ITU provides information related to the 5G technology trends, which is intended to address prominent issues such as significant improvement on system throughput, consistency of the user experience, scalability so as to support IoT, delay, energy efficiency, cost, network flexibility, support for new services and flexible spectrum utilization, etc.
The random access procedure, as an important step in a wireless communication system, is used for establishing uplink synchronization between a user equipment (UE) and a base station (BS), for allocating by the base station an identifier (ID) to the UE for identification of the UE, or the like. The performance of random access directly influences the use experience of UE. In conventional wireless communication systems, such as LTE and LTE-Advanced, the random access procedure is applied in many scenarios, for example, establishment of initial connections, cell handover, re-establishment of uplink connections, re-establishment of radio resource control (RRC) connections, or the like. And the random access is divided into contention-based random access and contention-free random access, depending upon whether the UE uses the preamble sequence resources exclusively or not. Since a preamble sequence is selected from the same preamble sequence resources during the attempt of establishment of an uplink connection by UEs in the contention-based random access, it may be possible for a plurality of UEs to select a same preamble sequence to be transmitted to the base station. Hence, a contention resolution mechanism becomes an important research aspect of random access. How to reduce the contention probability and how to rapidly resolve contentions that have already taken place are key indicators that influence the performance of random access.
The contention-based random access procedure in LTE-A consists of four steps, as shown in FIG. 2. In step 1, a UE randomly selects one preamble sequence from a preamble sequence resource pool and transmits the preamble sequence to a base station. The base station performs correlation detection on an access signal so as to identify the preamble sequence transmitted by the UE. In step 2, the base station transmits a random access response (RAR) to the UE. The RAR includes a random access preamble sequence identifier, a timing advance (TA) indication determined according to time delay estimation between the UE and the base station, a temporary cell-radio network temporary identifier (C-RNTI), and time-frequency resources allocated to the UE for a next uplink transmission. In step 3, the UE transmits a message 3 (Msg3) to the base station according to information included in the RAR. The Msg3 includes a terminal identifier, an RRC connection request, among the others, and the terminal identifier is unique to the UE and used for resolving contentions. In step 4, the base station transmits a contention-resolution identifier to the UE. The contention-resolution identifier is a terminal identifier of the UE which is the ultimate winner of the contention resolution. The UE upgrades the temporary C-RNTI to C-RNTI after detecting the identifier thereof, transmits an acknowledge (ACK) signal to the base station to implement the random access procedure, and waits for the scheduling of the base station. Otherwise, the UE may start a new random access procedure after a period of time delay.
For a contention-free random access procedure, the base station may allocate a preamble sequence to the UE since it has known the identifier of the UE. Hence, the UE does not need to randomly select a sequence before transmitting the preamble sequence, and instead, the UE uses an allocated preamble sequence. The base station may transmit a corresponding random access response after detecting the allocated preamble sequence, and the random access response includes timing advance, an allocation of uplink resources and other information. After receiving the random access response, the UE recognizes that the uplink synchronization is completed and waits for the further scheduling of the base station. Therefore, a contention-free random access procedure just comprises 2 steps: the step 1 is to transmit the preamble sequence, and the step 2 is to transmit the RAR.
A millimeter-wave communication is a possible key technology in 5G. By increasing the carrier frequency to the millimeter-wave bands, the available bandwidth may be greatly increased, and hence the transmission rate of the system may be greatly improved. For resistance of the properties of high fading and high loss in wireless channels in the millimeter-wave bands, a millimeter-wave communication system generally uses the beamforming technology, that is, the beam energy is concentrated in a certain direction by using a weighting factor. During wireless communication, the base station and the UE search for an optimal beam pair by means of polling or the like so that a received signal-to-noise ratio (SNR) on the UE side is maximized. Since the UE and the base station do not know the direction of the optimal beam pair when the initial connection is established, the random access in the millimeter-wave communication system faces great challenges. One possible way is as described in [Random Access in Millimeter-Wave Beamforming Cellular Networks: Issues and Approaches], where, in step 1, UE tries all possible beam pairs at the time of transmitting a preamble sequence to search for an optimal beam pair which may be used in the subsequent steps of random access. In this solution, although an optimal beam pair may be obtained in step 1 of the random access procedure, the time required to transmit and detect the preamble sequence in step 1 may be prolonged. Hence, there is a great room for improving the performance.
In conclusion, in order to further improve the competitiveness of the millimeter-wave communication system in 5G candidate technologies, it is necessary to propose a technical solution that effectively solves the performance-associated problems of the random access procedure in the millimeter-wave system and improves the performance of the random access procedure in the millimeter-wave communication system, and finally achieves a goal of providing shorter access time delay and better access experience for users on the UE side.